Biometric systems are widely used as means for increasing the convenience and security of personal electronic devices, such as mobile phones etc. Fingerprint sensing systems, in particular, are now included in a large proportion of all newly released consumer electronic devices, such as mobile phones.
Optical fingerprint sensors have been known for some time and may be a feasible alternative to e.g. capacitive fingerprint sensors in certain applications. Optical fingerprint sensors may for example be based on the pinhole imaging principle and/or may employ micro-channels, i.e. collimators or microlenses to focus incoming light onto an image sensor.
US 2007/0109438 describe an optical imaging system which may be used as a fingerprint sensor where microlenses are arranged to redirect light onto a detector. In the described imaging system, each microlens constitutes a sampling point and the microlenses are arranged close to each other to increase the image resolution. To avoid mixing of light received from adjacent microlenses, micro-channels or apertures are arranged between the microlenses and the detector.
However, to achieve a high-resolution sensor, the microlenses will have to be made small and be manufactured with high precision, making the manufacturing process complex and sensitive to variations, and a sensor of the described type comprising small microlenses will also be sensitive to spatial differences in transmissivity in any layer covering the sensor.
Accordingly, it is desirable to provide an improved optical fingerprint sensing device.